Sanguine Sacrament is an instant that costs XWW. This means you must have two White mana and enough mana to pay for X to make it worth casting. Already, this seems like a fair investment, as later in the game, you can pump quite a bit of mana into this card. Being at instant speed means you can potentially use this card gain yourself enough life in order to not be dealt lethal damage. Best of all, you gain twice as much life as you pay for X.
Now, to make this card "worth" casting, you need to be able to invest quite a bit of mana into X. However, there is one thing about this card that makes it potentially valuable: when you cast it, you put it at the bottom of your library instead of putting it in your graveyard. That's actually a really big deal. Because this card is an instant, you can use it early in the game when you may not otherwise have a play in order to gain some quick life, even if it's only 2 or 4 life. Every point of life matters in Magic.
While I hardly see this card being widely adopted in Standard, and especially not Modern, there is the possibility that this card does see play in a semi-competitive combo deck. A deck that can produce a good deal of mana can make great use of this card. The real place for Sanguine Sacrament to shine is in Commander (EDH), where life-gain is not only useful, but can be an asset. There are many win conditions in Magic that involve having a certain life total, and this is one way to potentially gain yourself enough life to satisfy them.
There's Felidar Sovereign, for example, which lets you win the game if you have 40 or more life at the beginning of your turn. Sanguine Sacrament being at instant speed means that you can cast this card at the end of an opponent's turn. It can be a quick and dirty way to gain enough life to let the Sovereign win you the game. It's not the only way to win with this card, but it's one of the more popular ones.
In Commander there's also a popular creature called Rhox Faithmender that doubles any lifegain effect that you may have while he's on the battlefield. Also, a popular enchantment called Sunbond gives its enchanted creature as many +1/+1 counters as points of life that you gain. Sanguine Sacrament definitely belongs in any deck that runs those cards.
So why is Sanguine Sacrament such a sleeper? It does something that's popular in casual Magic, and casual Magic players make up the majority of the player base. It can fit into a wide array of strategies, and while it's not a primary card to enable a strategy, it does complement them. Lifegain is always going to be good in Magic. Also, since it has the ability to either keep combo decks alive with its effect or even win the game by increasing your life total beyond a certain level, it's always going to have some demand.
While it's hard to see Sanguine Sacrament being an extremely expensive card, lifegain cards with widespread appeal always end up being worth over $1. Because it can be played multiple times over the course of a game, it's likely that a card like this breaches at least the $2 mark over the course of time.
Have any Magic cards that you think are worth sleeping on? Let us know in the comments!